The present invention generally relates to a bidirectional signal transmission circuit in a computer system or the like, and more specifically, to a bidirectional signal transmission circuit suitable for making such a signal transmission circuit compact and for operating this signal transmission circuit under low power consumption, that various types of signal transmission circuits involving a single end transmission circuit are employed in a mixture form.
In designing of a computer system, a care should be taken into a characteristic impedance of a signal line, as described in, for instance, "Designing Knowhow for Microcomputer Systems" written by Y. Hayashi and H. Tsuneda published by CQ publisher company in 1988, at pates 274 to 275. That is to say, in case that an actual distance of a transmission path for constituting a data bus becomes long, there is a problem of an occurrence of reflection. As a result, as shown in FIG. 7, an end circuit (resistor) is provided in the transmission path so as to prevent the signal reflection problem.
FIGS. 1A and 1B are schematic block diagrams for showing arrangements of the conventional signal transmission circuits.
As represented in FIG. 1A, as the signal transmission circuit used in the computer system, a transmission path 7a such as a wiring board is employed to connect a transmitter circuit 72a of a single circuit element (logic circuit element) 71a and one end circuit 73a, and to which receiver circuits 75a and 75b constructed of either one or plural logic circuit elements 74a and 74b are connected, to constitute a single end transmission circuit. Further, as shown in FIG. 1B, there is a double end signal transmission circuit in which transmission paths 70a and 70b are employed to connect transmitter circuits 72b, 72c constructed of a single logic circuit element 71b with two end circuits 73b, 73c, and also receiver circuits 75c to 75f constructed of a plurality of logic circuit elements 74c to 74f are connected thereto.
The conventional single end transmission circuit is described in, for example, "Microelectronics Packaging Handbook" written by Rao R. Tummala and Eugene J. Rymaszewski from Nikkei BP publisher.
In these transmission circuits, the signal transmission direction is one direction, whereas there is a certain case that a bidirectional signal transmission circuit for transmitting a signal along a bidirectional direction is used by connecting both of a plurality of transmitter circuits and a plurality of receiver circuits to the same transmission path. Since this bidirectional signal transmission circuit has various advantages that a total number of signal terminals of the logic circuit elements can be reduced and a total quantity of signal paths such as a wiring board and a signal cable can be decreased, such a bidirectional signal transmission circuit is widely utilized.
FIG. 2 is a schematic block diagram for showing an arrangement of another conventional bidirectional signal transmission circuit.
In this figure, reference numerals 61a and 61b denote logic circuit elements, and reference numerals 62a and 62b show end circuits. These circuit elements and circuits are connected with each other by way of a transmission path 60. Input/output circuits 63a and 63b of the respective logic circuit elements 61a and 61b are constructed of transmitter circuits 64a to 64d and receiver circuits 65a and 65b, which are connected to common input/output terminals 66a and 66b.
In this bidirectional signal transmission circuit, logic information is transmitted from IN (1) to OUT (2) and also from IN (2) to out (1). In any cases, the signal is transmitted via the transmission path 60 and the input/output terminals 66a and 66b.
Generally speaking, an amount of a current outputted from a transmitter circuit in a bidirectional signal transmission circuit becomes twice as that of the transmitter circuit employed in the single end transmission circuit shown in FIG. 1A. As a result, as illustrated in the bidirectional signal transmission circuit shown in FIG. 2, two sets of the respective transmitter circuits 64a to 64d employed in the input/output circuits 63a and 63b are connected in parallel with each other in order to reduce the current amount per one transmitter circuit. Otherwise, the high output transmitter circuit exclusively used in the bidirectional signal transmission circuit must be utilized so as to reduce the current amount. As a consequence, there is a problem that the chip area occupied by the transmitter circuits 64a to 64d becomes large in the logic circuit elements 61a and 61b. Also, there is another problem that power consumption per one transmission path becomes high.
In case that the transmitter circuits function as signal receiving ends, when the output impedances of the transmitter circuits 64a to 64d in the input/output circuits 63a and 63b of the logic circuit elements 61a and 61b are not sufficiently high values, a waveform rounding happens to occur in a signal, which is caused by the currents flowing through these transmitter circuits 64a to 64d. As a consequence, the output impedances of the transmitter circuits 64a to 64d which functions are the signal receiving ends, must be selected to be sufficiently high values.
However, generally speaking the reflection waves cased from the load (74a etc.) are propagated to the single end transmission circuit shown in FIG. 1A, and then may cause the reflection noise to be reduced, which is produced by the mismatching phenomenon between the output impedance thereof and the characteristic impedance of the transmission path 70a. As a consequence, the output impedance of the transmitter circuit 72a is designed to be substantially equal to, or smaller than the characteristic impedance of the transmission path.
In view of differences in the demands for the above-explained output current amount of the transmitter circuit and the output impedance, the same transmitter circuit as the transmitter circuit 72a employed in the single end transmission circuit shown in FIG. 1B, cannot be directly utilized as the transmitter circuits 64a to 64d of the bidirectional signal transmission circuit shown in FIG. 2.